Subscriber line concentrators (Remote Switching Units, RSU) or subscriber access networks (Access Networks, AN) are used to connect conventional analog and ISDN subscribers and private branch exchanges. Both represent switching-center-external devices that are connected via concentrating interfaces to the peripheral devices of the exchange.
In line with their respective definition, the subscriber line concentrators/subscriber access networks have clearly established mechanisms for controlling the subscriber signaling to the peripheral equipment of the switching system. Signaling facilities for controlling the subscriber line concentrators/subscriber access networks themselves are provided at the same time. These affect, for example, the connection of speech channels of the subscriber connection or of the extension to speech channels of the interface of the subscriber line connection concentrator/subscriber access network to the peripheral equipment of the switching node. The operating status of the subscriber connection and extension are also matched on this path between the subscriber line concentrator/subscriber access network and the switching system.
The exchange of subscriber signaling and control information is normally message-based in the manner specified by the interface definition. Typically, the complete interface is PCM-based and certain time slots are reserved for message-based transmission of subscriber/extension signaling and control information.
The V5.2 interface is an example of a concentrating interface between a subscriber access network and a local exchange. This provides that ISDN signaling, ISDN packet data on the D channel and message-based analog signaling (PSTN) are transmitted in time slots of the interface defined in accordance with definable standard predetermined time slots of the interface. For control of the subscriber access network, signaling protocols are provided that affect the forwarding of information through the subscriber access network, the matching of the port and PCM route states and also the substitute switching of failed signaling channels.
With regard to the definition, proprietary message-based concentrating interfaces can differ substantially from the specifications of the V5.2 standard. All message-based interfaces to subscriber line concentrators/subscriber access networks have something in common, they transmit subscriber signaling information (ISDN signaling messages, ISDN packet data, PSTN messages) as well as the control information for routing or for port status matching in certain time slots of a TDM-based interface.
The convergence of packet-based networks and TDM networks leads to a situation where conventional analog and ISDN subscribers and private branch exchanges have to be accessed using packet-based transmission methods.
A highly efficient access to the Internet can be achieved via xDSL routes to the subscriber connection or via cable networks. This makes sufficient bandwidth available in the subscriber area to be able to additionally handle narrowband conventional telephony/fax traffic via the same access line. From the point of view of the subscriber, additional connectability of conventional subscriber terminals and private branch exchanges should be possible. In addition, all the subscriber performance features known from the conventional PSTN/ISDN networks should still remain available. The user data necessary for switching is, however, transmitted packet-based, in contrast to the situation of a subscriber line concentrator/subscriber access network. The same applies to the signaling.
In this situation, the problem arises of connecting conventional subscribers or extensions (i.e. POTS, ISDN BA, ISDN PRI) that are switched via subscriber terminals (e.g. IAD of an xDSL route, set-top box of a cable network) with access to a packet network, in such a way that all conventional subscriber performance features (including D channel packet data) are available in principle. Furthermore, the reusability of the hardware and software functions of a conventional exchange should be retained. It should be possible to supply user data flows, preferably packet based, or, as an option, via TDM technology to the exchange, and transmit signaling information and ISDN-D channel packet data packet-based to the exchange.
According to the prior art, this problem is solved by controlling the aforementioned conventional subscribers as part of a subscriber access network. This has central components (such as IAT) that convert the narrowband user data flow to TDM technology and prepare signaling and user data flows in the form of a V5.2 interface. The user data flows are then processed in a conventional local exchange of the PSTN/ISDN network. The input and output flows of broadband traffic are controlled by suitable upstream multiplexers. The relevant conditions are schematically shown in FIG. 1.
Accordingly, a subscriber access network AN is shown that supplies information on a plurality of subscriber terminals to the peripheral devices of an exchange LE. A device IAD that supports specific functions of the xDSL transmission method is provided as a subscriber access device. In a multiplexer DSLAM, narrowband information is separated from broadband information and the xDSL route is closed. The speech and signaling information is applied, IP based, via an ATM network as a carrier of a device IAT that forms the head end of the subscriber access network AN. The latter passes the speech and signaling information via a V5.2 interface to a local exchange LE.
The advantages of the concept shown here are on the one hand the use of an existing concentrating interface and on the other hand the availability of all the subscriber performance features supported by the exchange. The decisive disadvantage on the other hand is in the use of TDM technology in an exchange operating in an environment of converging networks. There is also a deterioration of the speech quality due to the, sometimes avoidable, conversion of the user data flow between TDM technology with Codec G.711 and the packet-based transmission by means of a generally compressing Codec such as G.723.1.